Optically monitoring an alox fabrication process

ABSTRACT

A method of forming an insulator that passes through a metal substrate ( 302 ) comprising: anodizing a region ( 312   a   , 312   b   , 314,316   a   , 316   b   , 318, 320   a   , 320   b   , 322, 324   a   , 324   b ) of the substrate to form the insulator; illuminating the region with light ( 330 ); and determining if the light passes through the substrate at the region to determine if the insulator passes completely through the substrate.

FIELD

This application relates to inspection and process control forinterconnect substrates, more particularly, ALOX™ substrates.

BACKGROUND

Microelectronics packaging and interconnection technologies haveundergone both evolutionary and revolutionary changes to serve the trendtowards miniaturization in electronics equipment, which is now veryevident in military, telecommunications, industrial and consumerapplications. The trend has been driven by various forces includingspecialist requirements for size and weight as well as cost andaesthetics, which have led to various innovative developments inpackaging of integrated circuits and in connectivity on electronicssubstrates and circuit boards.

In a broad sense, “microelectronic packaging” can simply be viewed as away to interface an IC (or a die) with the “real” world of peripheralssuch as power sources (e.g., power supplies, batteries, and the like),input devices (e.g., keyboards, mouses, and the like), and outputdevices (e.g., monitors, modems, antennas, and the like). To do this,you need to connect the IC (or die) with the peripheral—basically, toget signals in and out of the IC, as well as to provide operating powerto the IC—and this is typically done with wires or conductive traces ona printed wiring board (PWB).

There are many examples (or subsets) of interconnect substrates, oneexample is the “interposer”. Generally, an interposer provideselectrical connections between an IC and a package, may perform a pitchspreading function, typically does not “translate” connection types(rather, has one connection type on both the “in” side and the “out”side), and often must provide a thermal management function.

A fundamental purpose of an interconnect substrate is, simply stated, toelectrically connect two electronic components with one another. If, forexample, you have a simple two terminal device (such as a simpleresistor having two leads) poking through two holes on a PWB toconductors on the underside of the PWB, this is relativelystraightforward, even if there is a conductive trace on the PWB whichneeds to pass under a body portion of the two terminal device (withoutconnecting to it). However, with more complex electronic devices havingmany terminals (for example, input/output (I/O) connections) it isinevitable that there needs to be many crossovers to effect complexrouting of signals (to a lesser extent, power). Solutions to thistopological problem is multilayer interconnect technology.

In multilayer interconnect technology, there are typically several metallayers (of conductive traces) separated from one another by layersdielectric material. (Kind of like a layer cake, or lasagna.) Multilayerinterconnect substrates with tens of alternating dielectric andconductive layers are not uncommon, and typically many layers are neededto effect complex routing schemes (schematically speaking, manycross-overs).

A key element in every multilayer interconnect technology is the“via”—an electrical connection between conductive traces of two adjacentmetal layers separated by a dielectric material.

In conventional multilayer substrate technologies a dielectric sheet isused as base material, in which the vias are formed using drilling(etching or punching) and hole plating process. (A via is kind of like ametal eyelet for shoelaces.)

In multilayer substrate technology one type of via is the “blind” viawhich extends through a given dielectric layer(s) to a conductive traceon an inner metal layer, rather than completely through the entiresubstrate. Another blind via may extend through the remaining dielectriclayers from a different position on the conductive trace, which could beuseful for pitch spreading, or simply for effecting complexinterconnections.

Vias provide electrical connectivity between conductive traces on twodifferent (typically adjacent) metal layers, and also can serve a rolein conducting heat away from an operating electronic device mounted onthe substrate. Typically, with a dielectric-based substrate (such as aceramic substrate), the vast bulk of the substrate is poor thermalconductivity ceramic material, in which case many vias can be formed andfilled to improve the thermal conductivity. ALOX™ substrate technologyis described in the following patents and publications: U.S. Pat. No.5,661,341; U.S. Pat. No. 6,448,510; U.S. Pat. No. 6,670,704;International Patent Publication No. WO 00/31797; and InternationalPatent Publication No. WO 04/049424, the disclosures of which areincorporated herein by reference.

ALOX™ substrate technology is a multilayer substrate technologydeveloped for microelectronics packaging applications. The ALOX™substrate technology does not require drilling and hole plating—the viais of solid full aluminum and the dielectric is of a high qualityceramic nature. The process is simple and low cost, and contains a lownumber of process steps. The ALOX™ substrate technology serves as a widetechnology platform, and can be implemented in various electronicspackaging applications such as for RF, SiP, 3-D memory stacks, MEMS andhigh power modules and components.

The starting material in the ALOX™ process is a conductive aluminumsheet. A first step in the process is masking the top and bottom of thesheet using conventional masking techniques and materials (for example,lithography and/or photoresist). Via structures are formed usinganodization of the sheet through the whole thickness of the sheet. Theexposed areas are converted into aluminum oxide, which is ceramic innature and a highly insulating dielectric material. The protectedunexposed areas remain as aluminum elements—the connecting vias.

In its simplest form, an ALOX™ interconnect substrate is formed byelectrochemical anodic oxidation of selected portions of an initiallyconductive valve metal (for example, aluminum, titanium, or tantalum)substrate resulting in areas (regions) of conductive (starting) materialwhich are geometrically defined and isolated from one another by areas(regions) of anodized (non-conductive, such as aluminum oxide, oralumina) isolation structures. “Vertical” isolation structures extendinto the substrate, including completely through the substrate.“Horizontal” isolation structures extend laterally across the substrate,generally just within a surface thereof. Anodizing from one or bothsides of the substrate can be performed to arrive at complexinterconnect structures.

In a more complex form, such as disclosed in U.S. Pat. No. 6,670,704,using this innovative process, a multilayer low cost ceramic board isformed. A complete “three metal layer” core contains an internalaluminum layer, top and bottom patterned copper layers with though viasand blind vias incorporated in the structure. The ALOX™ technologyoffers a very simple and low cost production process; excellent thermalperformance product, superior mechanical and electrical properties. TheALOX™ technology is illustrated in the following figures.

FIG. 1A illustrates a schematic process flow 100 for via formation in anALOX™ substrate, and the resulting via formed thereby, according to theprior art. Starting (a) with an aluminum layer or substrate 102, amasking material 104 such as photoresist is applied (b) and patterned(c) to form optionally islands 105 of photoresist. Then, the unprotectedaluminum is anodized (d), converting selected areas 106 of thelayer/substrate 102 into non-conducting aluminum oxide leaving vias 108of conducting aluminum.

Notice in step (d) that the anodizing proceeds partiallyanisotropically, extending slightly under the photoresist and alsotapering in width from thickest at the top and bottom surfaces of thesubstrate to thinner within the body of the substrate. In step (d),anodization proceeds from both sides of the substrate. (In a situationinvolving a layer rather than a substrate, anodization would proceedfrom only an exposed side of the layer.) The resulting aluminum oxide isporous.

The photoresist islands 105 are stripped (e), and pore filling material,such as a resin is diffused into the porous oxide regions of thelayer/substrate. For a substrate, resin for example can be diffused fromboth sides. (Theoretically, the substrate could be impregnated withresin before photoresist strip.) The result is an aluminum via 108extending completely through the substrate from one surface thereof tothe opposite surface thereof, and the via is isolated from other suchvias (not shown) by the insulating (and impregnated) aluminum oxidematerial 106. This is referred to by the assignee as the “core ofcores”.

Next, metal interconnect layers 110 of conductive traces (such ascopper) are applied (f), using conventional technology to achieve whatthe assignee refers to as a “core”, which is a 3 metal layer structure.The process illustrated generally in FIG. 1A is shown and described ingreater detail in U.S. Pat. No. 6,448,510.

FIG. 1B is a cross-sectional view of an ALOX™ substrate comprising acore having 3 metal layers. As illustrated therein, a substantiallyplanar aluminum sheet 122 having a nominal thickness T typically between125-250 μm (microns) is anodized to create regions 124 of modifiedaluminum oxide (Al2O3) bounding and defining a variety of aluminumstructures comprising (from left to right in the figure) an internalaluminum layer 126 (which can be used for power or ground), an aluminumvia 128 extending completely through the sheet from the top surface tothe bottom surface thereof, and blind/thermal vias 130 and 132. Theprocess illustrated generally in FIG. 1B is shown and described ingreater detail in U.S. Pat. No. 6,670,704.

GLOSSARY

Unless otherwise noted, or as may be evident from the context of theirusage, any terms, abbreviations, acronyms or scientific symbols andnotations used herein are to be given their ordinary meaning in thetechnical discipline to which the disclosure most nearly pertains. Thefollowing terms, abbreviations and acronyms may be used throughout thedescriptions presented herein and should generally be given thefollowing meaning unless contradicted or elaborated upon by otherdescriptions set forth herein. Some of the terms set forth below may beregistered Trademarks®.

-   ALOX™ A substrate technology (proprietary to Micro Components Ltd.    of Ramat-Gabriel, Israel) wherein the substrate is metal based, made    of a combination of aluminum metal and aluminum oxide based    dielectric material forming a multi layer interconnect substrate,    typically in a BGA format.    Aluminum Aluminium, or aluminum (Symbol Al)-   Ampere (A) is the SI base unit of electrical current equal to one    coulomb per second. It is named after Andre-Marie Ampere, one of the    main discoverers of electromagnetism.-   Angstrom (Å) a unit of measurement equal to 10 exp-10 meters    (0.0000000001 meter). 10 Å=1 nm (nanometer).-   array a set of elements (usually referring to leads or balls in the    context of semiconductor assembly) arranged in rows and columns-   assembly the process of putting a semiconductor device or integrated    circuit in a package of one form or another; it usually consists of    a series of packaging steps that include: die preparation, die    attach, wirebonding, encapsulation or sealing, deflash, lead    trimming/forming, and lead finish-   chip A portion of a semiconductor wafer, typically containing an    entire circuit which has not yet been packaged-   chip-scale package (CSP)—any package whose dimensions do not exceed    the die's dimensions by 20%-   die 1. a single chip from a wafer, 2. a small block of semiconductor    material containing device circuitry.-   die attach the assembly process step wherein the die is mounted on    the support structure of the package, for example, the leadframe,    die pad, cavity, or substrate-   die used synonymously with “chip”. Plural, “dies” or “dice”.-   earth (electrical) another name for “ground”-   IC or ICC short for Integrated Circuit, or Integrated Circuit Chip.-   Interconnect Substrate As used herein, an interconnect substrate is    a typically flat substrate used to connect electronic components    with one another and having patterns of conductive traces in at    least one layer for effecting routing of signals (and power) from    one electronic component to another, or to the outside world.    Typically, an interconnect substrate has many metallization layers    with the conductive traces, and vias connect selected traces from    one layer to selected traces of another layer.-   interposer an intermediate layer or structure that provides    electrical connection between the die and the package-   leadframe A metal frame used as skeleton support to provides    electrical connections to a chip in many package types.-   mask Broadly speaking, a mask is any material forming a pattern for    a subsequent process to selectively affect/alter certain areas of a    semiconductor substrate, and not others. Photoresist is a    commonly-used masking material which is applied to the substrate,    then washed off (stripped) after the desired process is completed.-   microelectronics The branch of electronics that deals with miniature    (often microscopic) electronic components.-   micron (μm) a unit of measurement equal to one millionth of a meter    (0.000001 meter); also referred to as a micrometer.-   mil a unit of measurement equal to 1/1000 or 0.001 of an inch; 1    mil=25.4 microns-   nanometer (nm) a unit of measurement equal to one billionth of a    meter (0.000000001 meter).-   package a container, case, or enclosure for protecting a (typically    solid-state) electronic device from the environment and providing    connections for integrating a packaged device with other electronic    components.-   photoresist or, simply “resist”. Photoresist (PR) is a    photo-sensitive material used in photolithography to transfer a    pattern from a mask onto a wafer. Typically, a liquid deposited on    the surface of the wafer as a thin film then solidified by low    temperature anneal. Exposure to light (irradiation) changes the    properties of the photoresist, specifically its solubility.    “Negative” resist is initially soluble, but becomes insoluble after    irradiation. “Positive” resist is initially insoluble, but becomes    soluble after irradiation. Photoresist is often used as an etch    mask. In the context of the present disclosure, photoresist may be    used as an oxidation mask.-   PWB short for printed wiring board. Also referred to as printed    circuit board (PCB).-   semiconductors 1. Any of various solid crystalline substances, such    as germanium or silicon, having electrical conductivity greater than    insulators but less than good conductors, and used especially as a    base material for computer chips and other electronic devices. 2. An    integrated circuit or other electronic component containing a    semiconductor as a base material.-   SI units The SI system of units defines seven SI base units:    fundamental physical units defined by an operational definition, and    other units, which are derived from the seven base units, including:    -   kilogram (kg), a fundamental unit of mass    -   second (s), a fundamental unit of time    -   meter, or metre (m), a fundamental unit of length    -   ampere (A), a fundamental unit of electrical current    -   kelvin (K), a fundamental unit of temperature    -   mole (mol), a fundamental unit of quantity of a substance (based        on number of        -   atoms, molecules, ions, electrons or particles, depending on            the substance)    -   candela (cd), a fundamental unit luminous intensity    -   degrees Celsius (° C.), a derived unit of temperature. t°        C.=tK−273.15    -   farad (F), a derived unit of electrical capacitance    -   henry (H), a derived unit of inductance    -   hertz (Hz), a derived unit of frequency    -   ohm (Ω), a derived unit of electrical resistance, impedance,        reactance    -   radian (rad), a derived unit of angle (there are 2π radians in a        circle)    -   volt (V), a derived unit of electrical potential (electromotive        force)    -   watt (W), a derived unit of power-   SIP short for ‘System-in-a-Package’. A SIP (or SiP) is a package    that contains several chips and components that comprise a    completely functional stand-alone electronic system (also acronym    for ‘Single-in-Line Package’—a through-hole package whose leads are    aligned in just a single row, but that definition is not used in the    description herein)-   SMD short for ‘Surface-Mount Device’-   SMT short for ‘Surface-Mount Technology’-   substrate 1. the base material of the support structure of an IC; 2.    the surface where the die or other components are mounted during    packaging; 3. the semiconductor block upon which the integrated    circuit is built-   surface-mount a phrase used to denote that a package is mounted    directly on the top surface of the board, as opposed to    ‘through-hole’, which refers to a package whose leads need to go    through holes in the board in order to get them soldered on the    other side of the board-   valve metal a metal, such as aluminum, which is normally    electrically conductive, but which can be converted such as by    oxidation to both a non-conductor (insulator) and chemical    resistance material. Valve metals include aluminum (Al, including Al    5052, Al 5083, Al 5086, Al 1100, Al 1145, and the like), titanium,    tantalum, also niobium, europium and like.-   via A metallized or plated-through hole, in an insulating layer, for    example, a substrate, chip or a printed circuit board which forms a    conduction path itself and is not designed to have a wire or lead    inserted therethrough. Vias can be either straight through (from    front to back surface of the substrate) or “blind”. A blind via is a    via that extends from one surface of a substrate to within the    substrate, but not through the substrate.-   Volt (V) A measure of “electrical pressure” between two points. The    higher the voltage, the more current will be pushed through a    resistor connected across the points. The volt specification of an    incandescent lamp is the electrical “pressure” required to drive it    at its designed point. The “voltage” of a ballast (for example    277 V) refers to the line voltage it must be connected to. A    kilovolt (KV) is one thousand volts.-   Voltage A measurement of the electromotive force in an electrical    circuit or device expressed in Volts. It is often taught that    voltage can be thought of as being analogous to the pressure (rather    than the volume) of water in a waterline.-   Watt (W) A unit of electrical power. Lamps are rated in watts to    indicate the rate at which they consume energy. A kilowatt is 1000    watts.-   Wavelength The distance between two neighboring crests of a    traveling wave. The wavelength of (visible) light is between about    400 and about 700 nanometers.

SUMMARY

Generally, the disclosure is described in the context of ALOX™ substratetechnology. The ALOX™ substrate technology employs area selectiveanodization of aluminum substrates for forming patterned anodized(oxidized) areas defining corresponding patterned electrically-isolatedaluminum conductive areas, such as vias extending through the substrate.Typically, a vertical isolation structure surrounding a via will bering-shaped.

As used herein, aluminum is exemplary of any number of “valve metal”starting materials that is initially a good electrical conductor, andwhich can be selectively converted to a non-conductive (insulating)material (such as, but not limited to aluminum oxide) by a process suchas (but not limited to) electrochemical anodic oxidation resulting inconductive areas (regions) which are defined and isolated from oneanother by the insulting areas (regions).

The inventors have noted that aluminum oxide (Al2O3) is relativelytransparent to light and that therefore, a fully oxidized zone (such asvertical isolation structure) can be visually observed and inspectedusing light transmission. The inspection process may be automated.

An aspect of some embodiments of the invention relates to providing amethod for process control in ALOX™ substrate fabrication. A step inALOX™ substrate fabrication is anodizing an aluminum panel, areaselectively, to form pre-designed zones (vertical isolation structures)in the panel, which are fully oxidized through the whole thickness ofthe panel. Methods are disclosed for monitoring this process step anddetermining when it is complete using visual inspection of transmittedlight through the oxidized zones. The inspection process may beautomated.

According to an embodiment of the disclosure, a method of making aninterconnect substrate comprises: providing a valve metal substrate;selectively anodizing the substrate to form vertical isolation areasthat extend completely through the substrate; and determining whetherthe vertical isolation areas have been fully formed by shining lightthrough the substrate. The valve metal substrate may be aluminum.

Light may be observed shining through the substrate, and a determinationmay be made that a given vertical isolation area is defined as fullyformed if it appears as a continuous area of light. The verticalisolation areas may extend through the substrate, surround and definevalve metal vias which extend through the substrate and which areelectrically isolated from other valve metal vias and from the body ofthe substrate, in which case a given vertical isolation area isdetermined to be fully formed if it appears as a continuous ring oflight surrounding a corresponding one of the valve metal vias.

Observing whether the vertical isolation areas are fully formed may beperformed during anodizing the substrate, and the process of forming thevertical isolation areas may be continued if they are determined to notbe fully formed, until they are fully formed.

According to an embodiment of the disclosure, apparatus for inspectingan interconnect substrate comprising a valve metal substrate having aplurality of vertical isolation areas extending completely through thesubstrate and defining a plurality of valve metal vias electricallyisolated from the body of the substrate, comprises: a light source forshining light through the substrate; and detectors for observing whetherthe vertical isolation areas are fully formed.

The substrate may be held (supported) by a scanning mechanism, such asan X-Y mechanism under computer control, or by a light table. Inspectionmay be performed using a microscope, and determining whether thevertical isolation areas are fully formed (analyzing images) may beperformed with the computer.

The apparatus may be capable of functioning while the substrate is in ananodizing bath. A first mirror may be provided for reflecting light froma light source external the bath, through the substrate, to a secondmirror for reflecting light passing through the substrate to externalthe bath. Means for moving the mirrors to scan the substrate may beprovided, and means for moving the substrate in the bath may beprovided, to effect scanning.

There is therefore provided in accordance with an embodiment of theinvention, a method of forming an insulator that passes through a metalsubstrate comprising: anodizing a region of the substrate to form theinsulator; illuminating the region with light; and determining if thelight passes through the substrate at the region to determine if theinsulator passes completely through the substrate.

Optionally, the method comprises determining a pattern for the lightthat passes through the substrate. Optionally, the method comprisesdetermining whether the pattern is satisfactory. Optionally, the methodcomprises determining deeming that the insulator is satisfactorilyformed when the pattern is satisfactory. Additionally or alternatively,the method comprises stopping anodization when the pattern issatisfactory.

In some embodiments of the invention, determining if the light passesthrough the region comprises determining during anodization. In someembodiments of the invention, determining if the light passes throughthe region comprises determining when anodization has stopped.

There is further provided in accordance with an embodiment of theinvention, a method of forming an insulator that passes through a metalsubstrate comprising: anodizing a region of the substrate to form theinsulator; illuminating the region with light; and stopping anodizationwhen a sufficient amount of light passes through the substrate at theregion. There is further provided in accordance with an embodiment ofthe invention, a method of making an interconnect substrate comprisingan insulated via that passes through the substrate, comprising formingan insulator in accordance with an embodiment of the invention thatcompletely surrounds a region of non-anodized metal.

In some embodiments of the invention, the metal is a valve metal.

There is further provided in accordance with an embodiment of theinvention apparatus for forming an insulator that passes through a metalsubstrate comprising: apparatus for anodizing a region of the substrateto form the insulator; a light source configured to illuminate theregion with light; and at least one detector positioned to receive lightthat passes through the substrate at the region.

Optionally the apparatus comprises a scanning mechanism that moves thesubstrate relative to the detector so that light passing through theregion is incident on the at least one detector.

Optionally, the scanning mechanism moves the substrate relative to thelight source so that light passing through the region is incident on theat least one detector. Additionally or alternatively, the scanningmechanism moves the substrate. In some embodiments of the invention, thescanning mechanism moves the light source. In some embodiments of theinvention, the scanning mechanism moves the at least one detector. Insome embodiments of the invention, the at least one detector comprises amicroscope. In some embodiments of the invention, the light source isconfigured to illuminate the substrate during anodizing.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made in detail to preferred embodiments, examples ofwhich may be illustrated in the accompanying drawing figures. Thefigures are intended to be illustrative, not limiting. Although thedisclosure is generally described in the context of these preferredembodiments, it should be understood that it is not intended to limitthe claims to these particular embodiments.

Certain elements in selected ones of the figures may be illustratednot-to-scale, for illustrative clarity. The cross-sectional views, ifany, presented herein may be in the form of “slices”, or “near-sighted”cross-sectional views, omitting certain background lines which wouldotherwise be visible in a true cross-sectional view, for illustrativeclarity.

Cross-hatching may or may not be used in cross-sectional views. If itis, close-spaced diagonal line cross-hatching is used to indicateinsulator and wide-spaced cross hatching is used to indicate conductor.

Elements of the figures are typically numbered as follows. The mostsignificant digits (hundreds) of the reference number correspond to thefigure number. For example, elements of FIG. 1 (FIG. 1) are typicallynumbered in the range of 100-199, and elements of FIG. 2 are typicallynumbered in the range of 200-299. Similar elements throughout thefigures may be referred to by similar reference numerals. For example,the element 199 in FIG. 1 may be similar (and possibly identical) to theelement 299 in FIG. 2. Throughout the figures, each of a plurality ofelements 199 may be referred to individually as 199 a, 199 b, 199 c, andthe like. Such relationships, if any, between similar elements in thesame or different figures will become apparent throughout thespecification, including, if applicable, in the claims and abstract.

FIG. 1A is a diagram of a process flow for via formation in an ALOX™substrate, and the resulting via formed thereby, according to the priorart.

FIG. 1B is a cross-sectional view of an ALOX™ substrate, according tothe prior art.

FIG. 2 is a diagram illustrating a process flow, according to anembodiment of the invention.

FIG. 3A is a cross-sectional view of a technique for inspecting anexemplary ALOX™ substrate, according to an embodiment of the invention.

FIG. 3B is a top view of a portion of the exemplary ALOX™ substrate ofFIG. 3A.

FIGS. 4A-4D are cross-sectional views of an ALOX™ substrate beingsubjected to two-sided anodization.

FIG. 5A is a photomicrograph of an ALOX™ substrate having fully-formedvia structures, in accordance with an embodiment of the invention.

FIG. 5B is a photomicrograph of an ALOX™ substrate havingpartially-formed via structures, in accordance with an embodiment of theinvention.

FIG. 6 is diagram illustrating an embodiment of an “off-line” techniquefor inspecting an exemplary ALOX™ substrate, according to an embodimentof the invention.

FIG. 7 is diagram illustrating an embodiment of an “in-situ” techniquefor inspecting an exemplary ALOX™ substrate, according to an embodimentof the invention.

DETAILED DESCRIPTION

The disclosure relates to inspection techniques for interconnectsubstrates, such as ALOX™ substrates.

FIGS. 1A and 1B, described hereinabove, disclose the ALOX™ technology,generally. As discussed hereinabove, the ALOX™ substrate starts with avalve metal (such as aluminum) substrate 102 which is initiallyconductive, and portions of the substrate are anodized in a controlledmanner to produce areas or regions 104 of valve metal oxide (such asaluminum oxide). Anodization can be performed from either one or bothsides of the substrate, and can proceed partially or fully through thesubstrate. In some cases, the anodization proceeds completely throughthe substrate, and may be ring-shaped so as to defineelectrically-isolated regions of aluminum extending as “vias” throughthe substrate. Anodized areas extending completely through the substrateare referred to herein as “vertical isolation regions”.

When aluminum is anodized, it becomes converted to aluminum oxide.Whereas aluminum is a good electrical conductor, aluminum oxide is anelectrically-insulting material, thus enabling a substrate of aluminum(valve metal) to be made into an interconnect substrate havingelectrically-isolated aluminum structures such as vias extending throughthe substrate by anodizing (converting to aluminum oxide) selected areasof the substrate.

The inventors have noted that for such a substrate (referred to hereinas an “ALOX™ substrate”) the anodized (aluminum oxide) areas arerelatively transparent to light. Thus, when an ALOX™ substrate is heldup to light, one can see evidence of the structures “buried” within thesubstrate. The aluminum portions of the substrate block light, andaluminum oxide portions extending completely through the substratetransmit light. Therefore, a vertical isolation ring (of aluminum oxide)surrounding a via (of aluminum) is readily observed as a ring of light.This phenomenon of having two materials in the substrate (aluminum,aluminum oxide) having different transparency, is used, in accordancewith some embodiments of the invention, for inspecting ALOX™ substrates,both during and after processing, as described herein below.

FIG. 2 illustrates an exemplary overall process flow 200 formanufacturing ALOX™ substrates.

In a first step 202, masking processes are performed. This includesphotolithography, dense anodizing and second masking. Generally, apattern of masking material, such as dense oxide mask, photoresist mask,is disposed on the surface(s) of the substrate, using conventionalphotolithography techniques, to prevent the areas covered by maskingmaterial from being anodized and, conversely, to allow the areas notcovered by masking material to be anodized, as is known. This step isperformed to mask the substrate, prior to anodization, using knownprocesses.

In a next step 204, the substrate is anodized, using known processes.

In a next step 206, the substrate is inspected, as described greaterdetail hereinbelow. (Generally, the purpose of inspection is todetermine whether the anodization is complete, and is based ontransmitted light inspection.) Good inspection is generally advantageousat this stage, because the following processes are expensive (porefilling, sputtering, lapping).

In a next step 208, post-anodization processes, such as coppermetallization (refer to FIG. 1B), are performed.

Generally, the technique(s) disclosed herein are directed at monitoringthe results of the anodizing step (204). Generally, a desired result isfull anodization completely through selected areas of the aluminumsubstrate, to form regions of electrical insulation surrounding andisolating conducting regions. The conducting and insulating regions mayhave any of various desired shapes. Optionally, the insulating regionsare ring-shaped aluminum oxide vertical isolation areas enclosing andelectrically isolating aluminum via structures extending through thesubstrate.

FIG. 3A schematically illustrates an exemplary ALOX™ substrate 302 beinginspected using light transmission. The substrate 302 is generallyplanar, having a top side and a bottom side (top and bottom, as viewed).The substrate 302 is illustrated as having (from left-to-right, asviewed):

-   -   an internal aluminum layer 304 (compare “internal aluminum        layer” in FIG. 1B);    -   an aluminum via 306 which extends completely through the        substrate 302 from the top surface thereof top the bottom        surface thereof;    -   a “hybrid” aluminum structure 308 comprising an aluminum via        portion (compare 306) extending completely through the substrate        302 and an internal aluminum layer portion (compare 304); and    -   an internal aluminum layer 310 (compare 304).

Note that the internal aluminum layer 304 is horizontally spaced apartfrom the aluminum via 306, the aluminum via 306 is horizontally spacedapart from the composite aluminum structure 308, and the compositestructure 308 is horizontally spaced apart form the internal aluminumlayer 310.

The remainder of the substrate 302 (those areas which are not aluminum)have been converted to aluminum oxide (to define the aluminum structuresdescribed hereinabove), as follows:

-   -   an area 312 a above the internal aluminum layer 304;    -   an area 312 b below the internal aluminum layer 304;    -   an area 314 between the internal aluminum layer 304 and the        aluminum via 306, and extending completely through the        substrate;    -   an area 316 a above a left portion of the internal aluminum        layer portion 308 of the composite aluminum structure 308; 304;    -   an area 316 b below the left portion of the internal aluminum        layer portion 308 of the composite aluminum structure 308;    -   an area 318 between the composite aluminum structure 308 and the        aluminum via 306, and extending completely through the        substrate;    -   an area 320 a above a right portion of the internal aluminum        layer portion 308 of the composite aluminum structure 308; 304;    -   an area 320 b below the right portion of the internal aluminum        layer portion 308 of the composite aluminum structure 308;    -   an area 322 between the composite aluminum structure 308 and the        internal aluminum layer 310, and extending completely through        the substrate;    -   an area 324 a above the internal aluminum layer 310; and    -   an area 324 b below the internal aluminum layer 310.

For purposes of this example, the substrate 302 is being inspected,after anodizing (step 202). Methods for making the exemplary substrate302, and the substrate itself, are known.

FIG. 3A further illustrates that light, from a suitable light source(not shown) is directed at a surface, in this example, a top surface 332of the substrate, as indicated by arrows 330 pointing down on the topsurface of the substrate. The light can be, but is not limited to,visible light.

An observer, looking at the opposite side, in this example, a bottomsurface 334 of the substrate 302 will see patterns of dark and light,corresponding to areas where there is aluminum (such as 304, 306, 308,310), and areas where there is aluminum oxide, respectively. This isreminiscent of looking at an X-ray, except that instead of seeing bones,the observer can see aluminum structures (and, the observer can view thestructures directly, rather than through the intermediary of film).

Notice in FIG. 3A that there are no aluminum structures between the topand bottom surface in the aluminum oxide areas 314, 318 and 322 whichare between horizontally spaced apart aluminum structures 304, 306, 308and 310, respectively.

Aluminum oxide (ceramic) is relatively transparent, as compared withaluminum (metal). A typical via structure, such as 306, has a shape thatis round (looking at it from either surface 332 or 334 of thesubstrate), surrounded by a ring of anodization (such as the areas 314and 318, which are contiguous with one another).

In ALOX™ technology, vias are typically round, and tend to have atapered shape, as illustrated in FIG. 3A. A via such as 306 will have asmall diameter at the surfaces of the substrate, then increase indiameter towards the interior of the substrate. This taper is generallya “byproduct” of the anodization process.

FIG. 3B is an illustration of what an observer will see when viewing thesubstrate 302 from the bottom side, with light shining onto substrate302 from the top side. If the anodization step 202 (FIG. 2) has beensuccessfully completed, and if the aluminum via structure 306 has around shape, the observer should see the following.

Regarding the via 306, the observer will be able to see the aluminum viastructure 306 as a substantially opaque (non-light transmissive,non-transparent, non-translucent) “dark” circle 346 surrounded by arelatively transparent “light” ring-shaped area 348. The dark circlecorresponds to the larger diameter of the via 306 within the substrate302. The observer may also be able to distinguish a smaller circle 350,which is the smaller diameter of the via 306 at the surface of thesubstrate 302.

The observer will also be able to see the inner edge of the internalaluminum layer 304 (which may be in the form of a ring 352 surroundingthe via 306).

The ability to observe light areas 348 corresponding to regions such as314 and 318 (FIG. 3A) surrounding dark areas (such as 306) allows forverification that the anodization process (step 202) has beencompleted—that there is “full anodization”. Or, as discussedhereinbelow, that anodization has not been successfully completed.

Again, the examples set forth herein are generally in the context ofround-shaped vias surrounded by rings of vertical isolation. Hence, fullanodization will be indicated by bright, continuous ring of lightsurrounding a dark circle.

Recalling that it is the areas which are not covered by masking material(step 202) that become anodized (step 204), hence relativelytransparent, it is evident that the inspection of the substrate forrings of light can be performed either before stripping the maskingmaterial, such as during the anodizing process (as describedhereinbelow), or after stripping the masking material, such as after theanodizing process (as described hereinbelow.

Generally, as is known, the purpose of a via in an interconnectsubstrate is to effect an electrical connection between an isolated areaon the top surface of the substrate and a corresponding isolated area onthe bottom surface of the substrate, and an ALOX™ substrate is nodifferent in this regard. In this regard, the purpose of the aluminumvia structure 306 is realized if the surrounding aluminum oxide verticalisolation area (the ceramic ring formed around the aluminum via 306) iscompletely formed—in this example, the areas 314 and 318 being fullyoxidized.

FIGS. 4A-4D schematically illustrate an ALOX™ substrate 402 beinganodized to form a plurality of conductive aluminum vias 406 (compare306) surrounded by a plurality of vertical isolation structures 414(compare 314, 318).

In FIG. 4A, the substrate 402 is shown with patterned masking material424 and 428 on a top surface 403 thereof, and patterned masking material426 and 430 on a bottom surface 404 thereof. Vias 406, which in FIG. 4Aare not yet formed but will be produced by the end of the anodizingprocess are schematically shown in dashed lines. In FIG. 4B-4C as theprocess progresses, portions of vias 406 that are formed are shown insolid lines.

Typically, for making vias (406, compare 306), the top and bottommasking patterns are identical. Areas, which are not intended to beanodized are masked (covered) by material 424 and 428. Areas covered bymask material 424 and 428 will become the aluminum vias 406 (not formedyet, shown with dashed lines). Masking material 424 and 426 determinesthe main bodies of vias 406 and remains throughout the anodizingprocess. In areas between the masking material 404, anodization willproceed, to form the vertical isolation structures 414 (not formed yet)which electrically isolate the vias 414 from the other vias 414 and fromthe body of the substrate 402. Masking material 428 and 430 is providedso that that during anodizing, rate of growth of anodized material intosubstrate 402 proceeds at a relatively same rate for most regionsbetween vias 406. Masking material 428 and 430 does not remainthroughout the anodizing process. The material is configured to dissolveand/or amalgamate with material anodized during the anodizing processand disappear, or to be removed, at about a time when anodizing haspenetrated for example from surface 403 or 404 to a depth of about 15%or 25% of the total thickness of substrate 402. Use of masking materialto control rate of growth of anodized material is described in U.S. Pat.No. 6,670,704 the disclosure of which is incorporated herein byreference.

Once masked, anodization can proceed using any of various suitableanodizing methods known in the art.

FIG. 4B schematically illustrates substrate 402 part way into theanodization process, such as 33% of the way to completion. Depth ofanodization and conversion of aluminum to aluminum oxide isschematically represented by depth of shading in the substrate. Here, itcan be seen that anodization has begun, making its way through thesubstrate 402 (from one surface to the other), as well as proceedinglaterally, under the masking elements 424, 426.

The anodization process is largely anisotropic, and will proceed notonly through the substrate 402, but also under the masking material 424,426, 428 and 430. The size (such as diameter) of a masking element 424,426 is generally approximately the same size as the resulting via 406(that is, the diameter of the via 406 within the substrate 402).

FIG. 4C illustrates the substrate part way into the anodization process,such as 67% (two thirds, ⅔) of the way to completion. Here, it can beseen that anodization has progressed further through the substrate 402(from one surface to the other), as well as further laterally, under themasking elements 424, 426.

The mask elements 424, 426 must be properly sized and the anodizationprocess controlled so that the anodization does not proceed laterallycompletely across the intended via 406 at the surface(s) of thesubstrate 402. Else, that would result in a “buried via” (resembling aninternal aluminum layer, such as 304) which does not emerge at thesurface of the substrate 402.

FIG. 40 illustrates the substrate 402, successfully (fully) anodizedexcept for relatively small islands 413 of un-anodized material toprovide isolation structures 414 (compare 314) extending completelythrough substrate 402 and surrounding the vias 406 (compare 306).Islands 413 are isolated islands of Aluminum optionally remaining at theend of the anodizing process when isolation structures are sufficientlyformed to provide desired isolation of vias 406. The masking material424, 426 can be stripped away, and copper metallization performed (seeFIG. 2).

In FIG. 4D we see vertical isolation structures 414 (compare 314)extending completely through the substrate 402 and surrounding the vias406 (compare 306).

An unsuccessful, or “partial” anodization may look something like thepartially complete anodization illustrated in FIG. 4C, at selectedspots.

FIG. 5A is a schematic photomicrograph of what an observer would seewith light shining through an ALOX™ substrate having a plurality(approximately 50 shown) of via structures (seen as dark spots)surrounded by fully-formed ring shaped vertical isolation regions (seenas white rings surrounding the dark spots). Of interest here is not onlythe presence of all the white rings, but also their uniformity.

To give the reader an idea of scale, the area being shown in FIG. 5Ameasures approximately 10×7 mm, the dark spots have a diameter ofapproximately 160 μm, the rings have an outer diameter of approximately320 μm and the dark spots are spaced approximately 1 mm apart from oneanother. The dark spots correspond to vias, such as 306 (FIG. 3A), 406(FIG. 4D). The light rings surrounding the dark spots correspond tofully formed vertical isolation rings, such as 314, 414 surrounding thevias. The overall substrate (only a portion of which is shown in FIG.5A) may measure 100 mm×100 mm.

FIG. 5B is a schematic photomicrograph of what an observer would seewith light shining through an ALOX™ substrate having a plurality of vias(dark spots) surrounded by vertical isolation structures indicated byrings of light, in this case, partially formed rings of light. In thisfigure, only 3 vias are shown (seen as dark spots) surrounded bypartially formed ring shaped isolation zones (seen as white ringssurrounding the dark spots). The via on the top left is nearly fullyformed, since the light colored ring surrounding the (dark colored) viais about 80% formed. The via on the top right is less fully formed,since the (light colored) ring surrounding the (dark colored) via isonly about 60% formed. And, the via at the middle bottom is even lessfully formed, since the (light colored) ring surrounding the (darkcolored) via is only about 40% formed. These are 3 examples of vias thatare not fully formed, because the anodization process for forming oxiderings around the vias has not completed, for one reason or another. Inother words, the vias are defective, which may make the substrateunusable.

To give the reader an idea of scale, the area being shown in FIG. 5Bmeasures approximately 2×2 mm, the dark spots have a diameter ofapproximately 50 μm, the (partially-formed) rings have an outer diameterof approximately 300 μm, and the dark spots are spaced approximately 1mm apart from one another.

All three vias schematically shown being inspected in FIG. 5B inaccordance with an embodiment of the invention are defective because thevertical isolation rings surrounding (and defining, and intended toelectrically-isolate) the vias are not fully formed (they are “partiallyformed”). If inspection is being made during the anodization process(step 204 FIG. 2), the process optionally continues until the verticalisolation rings are fully formed. If this inspection is made after theanodization process has been completed, it is possible that thesubstrate can be put back into the anodizing process so that thevertical isolation rings can continue to be formed until they are fullyformed. The process of inspecting an ALOX™ substrate may be automated,as follows.

FIG. 6 illustrates, schematically, a system 600 for “off-line”inspection of substrates. Substrates being inspected by system 600 areoptionally substrates that have already had the anodizing stepperformed, and are being inspected for successful formation of vias, andany structures which were intended to be formed in the substrate.

Generally, a substrate 602 under test is placed in an X-Y scanningmechanism, schematically represented by a rectangle 604, such as in aframe holding (supporting) the edges of the substrate, rather than on atable, so that light from a light source 606 can be directed at asurface (bottom, as viewed in the figure) of the substrate, and lightpassing through the substrate can be detected/observed by an opticalapparatus such as a microscope 608. With an optical apparatus such as amicroscope 608, X-Y scanning (in this example, moving the substrate) isneeded so that the entire substrate can be brought into the field ofview (FOV) of the microscope.

Alternatively, the substrate may be placed on (and supported by) a lighttable, such as of the type used to view photographic negatives, and thelight table may be stationary. If the light table is stationary, themicroscope (or other optical apparatus) can be fitted for X-Y motion sothat the entire surface of the substrate can be scanned—in this example,by moving the field of view across a stationary substrate.

In either case, an X-Y mechanism for moving the substrate or themicroscope while it is being inspected would be under the control of acomputer for controlling movement of the X-Y mechanism. The computer isalso capable of analyzing images of the substrate being inspected, usingany suitable matching algorithm, such as by comparing images totemplates stored in computer memory, for example, or more detailedanalysis of the rings of light, their intensity, their uniformity, theirdimensions, and any other similar criteria.

The X-Y “scanning” would generally be required if the light source emitsa beam of light, rather than a diffuse field of light, which illuminatesonly a portion of the substrate, so that the entire substrate may bescanned and inspected.

FIG. 7 illustrates, schematically, a system 700 for “in-situ” inspectionof substrates, such as for example a substrate 702 undergoing anodizingstep 204 (FIG. 2) performed, in a tank 704 of anodizing solution, underappropriate conditions for performing anodizing. The inspectionapparatus is capable of functioning during anodization.

Light is directed from a light source 706 external to the tank, down (asviewed) into tank 704, to a first mirror 708 within the tank, whichreflects the light, optionally at 90 degrees, onto a surface 703 (left,as viewed in the figure) of the substrate. Light passing through thesubstrate is reflected by a second mirror 710 to an optical apparatussuch as a microscope 712. Supports, indicated by small circles behindthe mirrors may be provided for moving the mirrors to facilitatescanning the substrate, such as by pivoting.

As in the previous (off line) example, some form of scanning may berequired. In this case, the mirrors can be scanning mirrors, capable ofrotating about appropriate axes so that the entire substrate may bescanned by a beam of light. Supports, indicated by small circles behindthe mirrors may be provided for rotating the mirrors to facilitatescanning the substrate, such as by pivoting. Alternatively and/oradditionally, the substrate itself can be moved to effect or augment“scanning”, such as being withdrawn from the bath (as illustrated). Forexample, the mirrors could control scanning left and right, while thesubstrate is moved to effect scanning up and down. To this end, means(such as a robotic device which clamps the substrate and inserts it intothe bath) may be provided for moving the substrate in the bath, toeffect at least one axis of scanning.

As in the previous (off line) example, the scanning mechanism would beunder the control of a computer, which also analyzes images of thesubstrate under inspection such as by comparing images to templatesstored in computer memory, for example.

The process disclosed herein may be conducted on large panels ofaluminum typically having thousands of isolated aluminum vias andoxidized zones distributed over the substrate area.

The inspection technique disclosed herein can be used in different modesto achieve various objectives, such as:

1) for end point detection for the process step (204) of anodizing:

-   -   a) using in-line (“in situ”) inspection (FIG. 6) where a light        source and detection system for the light pattern can be        incorporated in the anodizing apparatus    -   b) using off-line inspection (FIG. 5) where the substrate is        pulled out of the oxidizing bath for visual inspection to        monitor the anodization process progression and formation of the        fully oxidized zones according to the pre-designed pattern;        2) for process monitoring and quality control of the product        post anodization, as the substrate can be visually checked,        using same transmitted light method, according to a certain        pass/fail criteria;    -   a) for yield enhancement    -   b) for failure analysis    -   c) for process uniformity analysis

It will be apparent to those skilled in the art that variousmodifications and variation can be made to the techniques described inthe present disclosure. Thus, it is intended that the present disclosurecovers the modifications and variations of the techniques, provided thatthey come within the scope of the appended claims and their equivalents.

1. A method of forming an insulator that passes through a metalsubstrate comprising: anodizing a region of the substrate to form theinsulator; illuminating the region with light; and determining if thelight passes through the substrate at the region to determine if theinsulator passes completely through the substrate.
 2. A method accordingto claim 1 and comprising determining a pattern for the light thatpasses through the substrate.
 3. A method according to claim 2 andcomprising determining whether the pattern is satisfactory.
 4. A methodaccording to claim 3 and comprising deeming that the insulator issatisfactorily formed when the pattern is satisfactory.
 5. A methodaccording to claim 3 and comprising stopping anodization when thepattern is satisfactory.
 6. A method according to any of claims 1-5wherein determining if the light passes through the region comprisesdetermining during anodization.
 7. A method according to any of claims1-5 wherein determining if the light passes through the region comprisesdetermining when anodization has stopped.
 8. A method of forming aninsulator that passes through a metal substrate comprising: anodizing aregion of the substrate to form the insulator; illuminating the regionwith light; and stopping anodization when a sufficient amount of lightpasses through the substrate at the region.
 9. A method of making aninterconnect substrate comprising an insulated via that passes throughthe substrate, comprising forming an insulator in accordance with any ofclaims 1-8 that completely surrounds a region of non-anodized metal. 10.A method according to any of claims 1-9 wherein the metal is a valvemetal.
 11. Apparatus for forming an insulator that passes through ametal substrate comprising: apparatus for anodizing a region of thesubstrate to form the insulator; a light source configured to illuminatethe region with light; and at least one detector positioned to receivelight that passes through the substrate at the region.
 12. Apparatusaccording to claim 11 and comprising a scanning mechanism that moves thesubstrate relative to the detector so that light passing through theregion is incident on the at least one detector.
 13. Apparatus accordingto claim 12 wherein the scanning mechanism moves the substrate relativeto the light source so that light passing through the region is incidenton the at least one detector.
 14. Apparatus according to claim 12 orclaim 13 wherein the scanning mechanism moves the substrate. 15.Apparatus according to any of claims 12-14 wherein the scanningmechanism moves the light source.
 16. Apparatus according to any ofclaims 12-15 wherein the scanning mechanism moves the at least onedetector.
 17. Apparatus according to any of claims 11-16 wherein the atleast one detector comprises a microscope.
 18. Apparatus according toany of claims 11-17 wherein the light source is configured to illuminatethe substrate during anodizing.